Many methods of preserving pulp yield in processes used for the alkaline pulping of lignocellulosic materials have been described. Most of these methods attempt to preserve pulp yield by either reducing or oxidizing the aldehyde end groups of the polysaccharide chains to prevent the alkaline peeling reaction [1]. Sodium borohydride [2], hydrogen sulphide [3], polysulphide [4,5] and anthraquinone [6] are examples of agents that have been used to provide higher yield in alkaline pulping processes.
Calcium hydroxide has been claimed to be able to increase the yield for kraft-AQ pulping [7]. In another claim [8], a portion of the black liquor produced in the pulping process was treated with lime under certain conditions and reused as cooking liquor. The effectiveness of generated polysulphide was enhanced by the carryover of soluble calcium. However in this reported work the function of the calcium was to increase the effect of AQ or PS; no yield increase was observed when calcium was used alone.
Calcium compounds have been proposed to treat hydrocellulose materials with cyanides to increase the yield [9]. It was found that the yield increase was due to the combined presence of cyanide and calcium chloride, and not one of these reagents alone. No indication of an increase in yield by impregnating wood with calcium chloride alone was identified in that work [9].
Calcium has also been proposed to stabilize hydrocellulose structures [10]. The technique is said to decrease the degradation of wood cellulose in a sodium hydroxide refining procedure but not a pulping process.
Although for a completely different purpose, impregnation of lignin solubilising agents, which include alkaline earth metal salts, into chips before pulping has been proposed [11]. However the boiling point of the solvent (a mixture of water and an organic solvent) proposed for impregnation had to be higher than the cooking temperature (160°-180° C.). The possibility of using water without an organic solvent for the impregnation is not described.
The impregnation of magnesium or calcium salts into wood chips in preparation for an oxidative soda-oxygen pulping process has been proposed [12]. The pulping process must be one in the absence of any sulphur form (elemental sulphur or hydrogen sulphide). The oxidative environment in the absence of sulphur makes it possible for the magnesium salt to be used as a protector.
This latter technique differs fundamentally from calcium impregnation followed by a reductive alkaline pulping, for example, a kraft pulping process which requires the presence of various forms of sulphur. Impregnation with a magnesium salt has a negative effect on yield in a reductive alkaline pulping process such as kraft.